Wind turbine blade with asymmetrical spar caps

ABSTRACT

A wind turbine blade ( 10 ) with asymmetric spar caps ( 36,38 ). The blade includes a pressure side spar cap ( 36 ) having pressure side fibers ( 56 ) having a pressure side fiber diameter ( 54 ), the pressure side fibers configured to resist a first flap deflection ( 20 ) in a first direction via tensile strength; and a suction side spar cap ( 38 ) having suction side fibers ( 50 ) having a suction side fiber diameter ( 52 ), the suction side fibers configured to resist the first flap deflection via a compressive strength. At a radial cross section, the suction side spar cap exhibits a greater compressive strength and the pressure side spar cap, for example, by the suction side fibers having a different compressive strength than the pressure side fibers.

FIELD OF THE INVENTION

The present invention relates to wind turbine blades. In particular, theinvention relates to spar caps having improved resistance to flapdeflection.

BACKGROUND OF THE INVENTION

Wind turbine blades rotate about a rotor hub of a wind turbine as aresult of aerodynamic forces created by relative wind passing over theairfoil surfaces of the blade. The airfoil surfaces include a pressureside and a suction side. Some of the relative wind encounters thepressure side and imparts force normal to the pressure side via amomentum of the relative wind. Some of the remaining relative windtraverses the suction side of the blade and increases in velocity as itdoes so. A velocity difference between the increased velocity on thesuction side and a velocity of air on the pressure side creates asuction force normal to the suction side. The pressure side force andthe suction side force combine to form a net aerodynamic force having anaerodynamic force direction that is the same as or close to thedirections of the suction side and pressure side forces.

Each point of a rotating wind turbine blade experiencing no aerodynamicforces would rotate in a respective theoretical plane of rotation.However, the wind turbine blade is not perfectly rigid and as a resultthe blade tends to deflect in a flap wise direction, which may be thesame or similar to the aerodynamic force direction. The amount ofdeflection of each point on the blade from that point's location in therespective theoretical plane of rotation increases from a base of theblade to a tip of the blade. This occurs because the base of the bladeis fixed to the rotor hub, while the deflections cumulate in theradially outward direction.

As technology advances, lengths of the blades increase. As the lengthsof the blades increase, the amount of flap deflection also increases.However, too much flap deflection may result in the blade contacting atower that supports the wind turbine. Consequently, flap deflection mustbe controlled.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention is explained in the following description in view of thedrawings that show:

FIG. 1 is a perspective schematic view of a wind turbine blade.

FIG. 2 is a cross section along A-A of one embodiment of the windturbine blade of FIG. 1.

FIG. 3 is a cross section along A-A of an alternate embodiment of thewind turbine blade of FIG. 1.

FIG. 4 is a side view of the wind turbine blade of FIG. 1.

FIGS. 5-11 are cross sections of various exemplary embodiments of sparcaps of the wind turbine blade of FIG. 1.

DETAILED DESCRIPTION OF THE INVENTION

The present inventor has recognized that a blade's resistance to flapdeflection can be improved by incorporating a relatively morecompression-resistant spar cap in the suction side of a wind turbineblade, and a relatively more tension resistant spar cap in the pressureside of the wind turbine blade.

As can be seen in FIG. 1, a turbine blade 10 may incorporate astructural spar having a spar cap 12 to reinforce each side of the blade10, and a spar web (not shown) spanning between and holding the sparcaps in a spaced apart relationship. The spar cap 12 may be a memberthat extends from a base 14 of the blade 10 toward a tip 16 of the blade10. The spar cap 12 may be incorporated into or otherwise secured to askin (shell) 18 that forms an aerodynamic shape of the blade 10.

As can be seen in FIG. 2 the spar (not shown) acts to resist flapdeflection 20 of the blade 10 from a plane of rotation 22 in a firstdirection that traverses the plane of rotation 22 from a pressure side24 of the blade toward a suction side 26 of the blade. FIG. 3 is a crosssection of the blade 10 of FIG. 1, taken along line A-A and showing thepressure side 24 of the blade 10 having a pressure side skin 28, thesuction side 26 of the blade 10 having a suction side skin 30, and oneconfiguration of the spar 32 having a spar web 34, a pressure side sparcap 36, and a suction side spar cap 38. Flap deflection 20 may occur asshown, and the flap deflection 20 shown would put the pressure side sparcap 36 in tension and the suction side spar cap 38 in compression. Thespar web 34 serves to hold the pressure side spar cap 36 and the suctionside spar cap 38 in a spaced-apart relationship. This increases theresistance of the spar 32, which acts like an “I-beam”, to flapdeflection 20. FIG. 4 shows a cross section of the blade 10 of FIG. 1,taken along line A-A, showing another configuration of a spar 40 havingtwo spar webs 42 connecting ends of the pressure side spar cap 44 andthe suction side spar cap 46. Any blade configuration having astructural support meant to resist flap deflection, and having a portionsubject to tension and a portion subject to compression during the flapdeflection, would be suitable for the structure disclosed herein.

Historically, the spar and spar caps were designed to provide adequatestrength for the blade so that it would simply withstand operatingstresses, such as centrifugal force, and spars have been symmetricalfrom pressure to suction side. As the blades have lengthened, theincreased flap deflection of a blade tip has increased to a point wherethe blade tip could collide with the tower during operation, even in ablade that is structurally sound in terms of strength. As a result,stiffness is becoming a limiting design criteria, and spars and sparcaps are being designed that are stronger in tension than a minimumrequired to resist operational forces such as centrifugal forces inorder to provide the stiffness required to prevent collision with thetower. The present inventor has innovatively taken advantage of thisrelationship to develop an improved overall blade design as describedmore fully below.

Conventional turbine blade spars have historically been designed to besymmetrical using reinforcing fibers throughout the spar caps and sparweb having fibers of one diameter and one material type. As used herein,a fiber diameter is an average diameter of the fibers in the spar cap,as individual fibers may vary in diameter due to manufacturingtolerances. In certain instances the average fiber diameter has beenlimited to not greater than 20 micrometers due to industry standardsestablished by Germanischer Lloyd in cooperation with the Wind EnergyCommittee as of Jul. 1, 2010. Other diameters were only permitted uponverification of a safe design. Consequently, until now, the designs haveresulted in pressure side spar caps having comparable cross sections tosuction side spar cap cross sections at any given radial location. Inother words, the pressure side spar caps and the suction side spar capsexhibited the same tensile strength and compressive strength at a givencross section.

For any given blade 10, both the pressure side spar cap 36 and thesuction side spar cap 38 may vary in shape and orientation from the base14 of the blade 10 to the tip 16 of the blade 10. However, for any givenradial location, in conventional blades a cross section of the pressureside spar cap 36 and the suction side spar cap 38 have been comparablein terms of compressive strength exhibited. The inventor proposes tochange this such that for any given radial cross section, the suctionside spar cap 38 has a greater compressive strength than does thepressure side spar cap 36. For a given set of rigidity requirements,this will allow for a lighter suction side spar cap than prior artdesigns

During flap deflection the pressure side is in tension and the suctionside is in compression. Reinforcing fibers used in the spar caps have acompressive strength that may be comparable to the tensile strength, butthe compressive strength is often not realized because the fibersthemselves tend to buckle in compression before realizing their fullcompressive strength. When in a spar cap, the fibers are held inalignment by matrix material and therefore buckling is hindered, andthus the compressive strength of the reinforcing fibers contributessignificantly to a compressive strength of the spar cap.

Since the pressure side fibers are in tension during flap deflection,buckling is not an issue, and they will be much more likely to reachtheir full tensile strength before breaking. However, the presentinventor has recognized that the ability of the matrix material to holdthe compression side fibers in alignment is limited, and as a result,the compression side fibers are likely to buckle before reaching theirfull compressive strength, and before the pressure side fibers reachtheir full tensile strength. Consequently, the suction side spar cap ismore likely to fail than the pressure side spar cap. The presentinventor exploits this fact by making an improvement in the compressivestrength of the suction side an important design goal.

This invention presents an innovative strategy for improving theresistance to flap deflection based on tailoring the suction side sparcap to improve its compressive strength. Such an approach, where thepressure side spar cap 36 and the suction side spar cap 38 areasymmetric, is contrary to the prior art turbine blades. Several ways toimprove the compressive strength of the suction side spar cap 38 exist.Those ways can be grouped into fiber-related improvements, nonfiber-related improvements, and any combination thereof.

Fiber related improvements acknowledge the fact that the reinforcingfibers have a greater compressive strength than the matrix material, butvirtually no resistance to buckling without the matrix material. Inturn, however, the matrix material can offer a certain resistance tofiber buckling. A fiber with a greater compressive strength will tend tobuckle at a higher compressive load, and so for a given matrix material,the spar cap using fiber with the greater compressive strength will beable to withstand a greater compressive load before buckling. Thus,while the matrix material may not be able to hold the stronger fibers inalignment until they reach their full compressive strength, it will holdthe stronger fibers in alignment until the suction side spar cap 38reaches a greater compressive load than would a suction side spar cap 38having fibers with a lower compressive strength.

One way to increase a compressive strength of the fiber, and thereforethe spar cap having the stronger fiber, is to increase a diameter of thefiber. For example, a single fiber from one roving, (a roving is a largenumber of roughly parallel fibers bundled together, twisted oruntwisted), having a diameter of approximately 18 micrometers, may havean E-modulus of approximately 79.0 GPa. The E-modulus is associated withthe compressive strength of the fiber. A fiber with a diameter of 24micrometers may have an E-modulus of approximately 89.0 GPa. Thus, anincrease of 6 micrometers in diameter may represent a 1.2% increase inthe E-modulus, and an associated increase in the compressive strength.Greater increases in the diameter may represent greater increases in theE-modulus and the associated compressive strength.

FIGS. 5-11 show a cross section of the pressure side spar cap 36compared to a cross section of the suction side spar cap 38 at any givenradial location. FIG. 5 shows a cross section of an exemplary embodimentof the suction side spar cap 38 where suction side fibers 50 have agreater diameter 52 than a diameter 54 of pressure side fibers 56. Inthis exemplary embodiment all of the suction side fibers 50 have thesame diameter 52 and all of the pressure side fibers 56 have the samediameter 54. In conventional blades all fiber diameters may be 20micrometers. In the exemplary embodiment of FIG. 5, the pressure sidefibers diameter 54 may still be 20 micrometers, but the suction sidediameter 52 may be any size larger than 20 micrometers. In an exemplaryembodiment the suction side diameter 52 may fall within a range of 25micrometers to 34 micrometers, inclusive. The suction side diameter 52may range much higher as necessary, up to and including 100 micrometers.Final diameters will be determined when considering all factors for eachapplication.

FIG. 6 shows a cross section of another exemplary embodiment of thesuction side spar cap 38 where the suction side fibers 50 include pluraldifferent diameters.

Each fiber having a distinct diameter may be considered a differentfiber type. Therefore, the suction side spar cap 38 may have a pluralityof fiber types. A first fiber type 60 may have a first diameter 62 and asecond fiber type 64 may have a second fiber diameter 66. There may beany number of fiber types in both the pressure side spar cap 36 and thesuction side spar cap 38, so long as a mixture of types of fibers in thesuction side spar cap 38 yields a greater compressive strength than amixture of types of fibers in the pressure side spar cap 36. Forexample, the first diameter 62 may be the same as the pressure sidefiber diameter 54 and the second diameter 66 may be greater than thepressure side fiber diameter 54. Alternately, both the first diameter 62and the second diameter 66 may be greater than the pressure side fiberdiameter 54. It is also conceivable that the first diameter 62 could besmaller than the pressure side fiber diameter 54 and the second diameter66 may be so much greater than the pressure side fiber diameter 54 as toyield an overall greater compressive strength of the suction side sparcap 38. Any mixture of diameters is possible so long as the crosssection of the suction side spar cap 38 ends up having a greatercompressive strength that the cross section of the pressure side sparcap at the same radial location.

Another way to increase a compressive strength of a fiber is to change acomposition of the fiber to a composition stronger in compressivestrength. For example, a carbon fiber has a greater compressive strengththan a glass fiber. FIG. 7 shows a cross section of another exemplaryembodiment of the spar caps 36, 38 where the suction side fibers 50include a stronger composition than do the pressure side fibers 56. Inthis exemplary embodiment all of the suction side fibers 50 have a samecomposition 68 as each other and all of the pressure side fibers 56 havethe same composition as each other. For example, the suction side fibers50 may be carbon fibers, which are stronger than glass fibers, while thepressure side fibers 56 may be glass fibers. Another type of fiber mayinclude aramide fibers. Any composition may be used in such an exemplaryembodiment, so long as the composition of the suction side fibers 50 hasa greater compressive strength than the composition of the pressure sidefibers 56.

FIG. 8 shows a cross section of another exemplary embodiment of the sparcaps 36, 38 where the suction side fibers 50 include plural differentcompositions. Each fiber having a distinct composition may be considereda different fiber type. Therefore, the suction side spar cap 38 may havea plurality of fiber types. A first fiber type 70 may have a firstcomposition 72 and a second fiber type 74 may have a second composition76. There may be any number of fiber types in both the pressure sidespar cap 36 and the suction side spar cap 38, so long as a mixture oftypes of fibers in the suction side spar cap 38 yields a greatercompressive strength than a mixture of types of fibers in the pressureside spar cap 36. For example, the first composition 72 may be the sameas a composition 78 the pressure side fibers 56 and the secondcomposition 76 may be a composition having a greater compressivestrength. Alternately, both the first composition 72 and the secondcomposition 76 may have a greater compressive strength than thecomposition 78 of the pressure side fibers 56. It is also conceivablethat the first composition 72 may have a weaker compressive strength andthe second composition 76 may be so much greater than the composition 78the pressure side fibers 56 as to yield an overall greater compressivestrength of the suction side spar cap 38. Any mixture of compositions ispossible so long as the cross section of the suction side spar cap 38ends up having a greater compressive strength that the cross section ofthe pressure side spar cap at the same radial location.

FIG. 9 shows a cross section of another exemplary embodiment of the sparcaps 36, 38 having a plurality of fiber types, where each fiber type hasa distinct combination of fiber diameter and fiber composition. Thesuction side spar cap 38 may have plural fiber types. There may be, forexample, the first diameter 62, the second diameter 64, and a thirddiameter 80. There may be the first composition 72, and the secondcomposition 76. A first fiber type 90 may have the first diameter 62 andthe first composition 72. A second fiber type 92 may have the firstdiameter 62 and the second composition 76. A third fiber type 94 mayhave the second diameter 64 and the first composition 72. A fourth fibertype 96 may have the second diameter 64 and the second composition 76. Afifth fiber type 98 may have the third diameter 80 and the firstcomposition 72. A sixth fiber type 100 may have the third diameter 80and the second composition 76. The number of fiber types is unlimited.The suction side spar cap 38 and the pressure side spar cap 36 each caninclude any mixture of fiber types, so long as the cross section of thesuction side spar cap 38 ends up having a greater compressive strengththat the cross section of the pressure side spar cap at the same radiallocation.

FIG. 10 shows a cross section of another exemplary embodiment of thespar caps 36, 38 where a matrix material 110 in the suction side sparcap 38 has a compressive strength greater than a compressive strength ofmatrix material 112 in the pressure side spar cap 36. Increasing thecompressive strength of the matrix material itself will contribute tothe compressive strength of the suction side spar cap 38. The matrixmaterial 110 in the suction side spar cap 38 may be the only differencebetween the pressure side spar cap 36 and the suction side spar cap 38.Alternately, adding matrix material 110 in the suction side spar cap 38having the greater compressive strength may be done in conjunction withany other technique described herein.

FIG. 11 shows a cross section of another exemplary embodiment of thespar caps 36, 38 where a cross sectional area 120 of the suction sidespar cap 38 is greater than a cross sectional area 122 of the pressureside spar cap 36 at the same radial location. Increasing the crosssectional area 120 of the suction side spar cap 38 will necessarilyresult in a suction side spar cap 38 with a greater compressivestrength. The increased cross sectional area 120 may be the onlydifference between the pressure side spar cap 36 and the suction sidespar cap 38. Alternately, increasing the cross sectional area 120 of thesuction side spar cap 38 may be done in conjunction with any othertechnique described herein.

From the foregoing it is apparent that the inventor has broken withconvention in order to tailor the design of the suction side spar cap tobetter meet the load conditions specific to the suction side of theblade. This individualized tailoring provides a suction side spar capwith a reduced weight, a greater compressive strength, or a combinationof both when compared to the pressure side spar cap, and when comparedto prior art suction side spar caps of similar stiffness requirements.This greater improved design allows for a lighter blade design toachieve a similar compressive strength, and the lighter blade may reduceforces and increase the life of the blade. It therefore represents animprovement in the art.

While various embodiments of the present invention have been shown anddescribed herein, it will be obvious that such embodiments are providedby way of example only. Numerous variations, changes and substitutionsmay be made without departing from the invention herein. Accordingly, itis intended that the invention be limited only by the spirit and scopeof the appended claims.

The invention claimed is:
 1. A wind turbine blade comprising: a pressureside spar cap comprising pressure side fibers comprising a pressure sidefiber diameter, the pressure side fibers configured to resist a firstflap deflection of the blade in a first direction via tensile strength;and a suction side spar cap comprising suction side fibers comprising asuction side fiber diameter, the suction side fibers configured toresist the first flap deflection via a compressive strength; wherein ata radial cross section the suction side fibers comprise a differentcompressive strength than the pressure side fibers.
 2. The wind turbineblade of claim 1, wherein the suction side fibers exhibit a greatercompressive strength than the pressure side fibers.
 3. The wind turbineblade of claim 1, wherein the pressure side fibers comprise fibers of afirst diameter, the suction side fibers comprise fibers of a seconddiameter, and wherein the second diameter is greater than the firstdiameter.
 4. The wind turbine blade of claim 3, wherein the seconddiameter is greater than 20 micrometers.
 5. The wind turbine blade ofclaim 3, wherein the second diameter falls within a range from 25micrometers to 34 micrometers, inclusive.
 6. The wind turbine blade ofclaim 1, wherein the pressure side fibers comprise a first mixture offiber types, each type comprising a distinct diameter, and wherein thesuction side fibers comprise a second mixture of fiber types differentthan the first mixture of fiber types.
 7. The wind turbine blade ofclaim 1, wherein the pressure side fibers comprise fibers of a firstcomposition and the suction side fibers comprise fibers of a secondcomposition different than the first composition.
 8. The wind turbineblade of claim 1, wherein the first fiber composition comprises glassfiber, and wherein the second fiber composition comprises carbon fiber.9. The wind turbine blade of claim 1, wherein the pressure side fiberscomprise a first mixture of fiber types, each type having a distinctcomposition, and wherein the suction side fibers comprise a secondmixture of fiber types different than the first mixture of fiber types.10. The wind turbine blade of claim 1, wherein the suction side fibersand the pressure side fibers comprise plural fiber types, wherein eachof the plural fiber types comprises a distinct combination of diameterand composition, wherein the pressure side fibers comprise a firstmixture of fiber types, and wherein the suction side fibers comprise asecond mixture of fiber types different than the first mixture of fibertypes.
 11. A wind turbine blade, comprising: a spar, comprising: apressure side spar cap comprising pressure side fibers configured toresist a first flap deflection in a first direction; a suction side sparcap comprising suction side fibers configured to resist the first flapdeflection; a spar web configured to hold the pressure side spar cap andthe suction side spar cap in a spaced-apart relationship; a pressureside shell comprising the pressure side spar cap; and a suction sideshell comprising the suction side spar cap, wherein the pressure sidefibers and the suction side fibers comprise at least one of a pluralityof different diameters and a plurality of different compositions, andwherein at a given radial cross section the suction side fibers comprisea greater compressive strength than the pressure side fibers.
 12. Thewind turbine blade of claim 11, wherein a diameter of the pressure sidefibers is less than or equal to 20 micrometers, and a diameter of thesuction side fibers are greater than 20 micrometers.
 13. The windturbine blade of claim 11, wherein the pressure side spar cap comprisesglass fibers and the suction side spar cap comprises carbon fibers. 14.The wind turbine blade of claim 11, wherein the pressure side spar capcomprises a first mixture of carbon fibers and glass fibers and thesuction side spar cap comprises a comprises a second, different mixtureof carbon fibers and glass fibers.
 15. A wind turbine blade, comprising:a pressure side spar cap comprising pressure side fibers; and a suctionside spar cap comprising suction side fibers; wherein for a given radiallocation along the blade, cross sections of the pressure side spar capand the suction side spar cap are asymmetric; and wherein the suctionside cross section comprises a greater compressive strength than thepressure side cross section.
 16. The wind turbine blade of claim 15,wherein the suction side cross section comprises a larger area than thepressure side cross section.
 17. The wind turbine blade of claim 15,wherein the suction side fibers comprise a larger diameter than thepressure side fibers.
 18. The wind turbine blade of claim 15, wherein acomposition of the suction side fibers comprises a greater compressivestrength than a composition of the pressure side fibers.
 19. The windturbine blade of claim 15, wherein matrix material in the suction sidecross section comprises a greater compressive strength than matrixmaterial in the pressure side cross section.